Qur'an, in Arabic, could only have been written by ONE of 3 possible sources:

1 The Arabs
2 Mohammad (Peace Be Upon Him)
3 Allah (GOD)

There are hundreds of religions flourishing around the world: Christianity, Islam, Buddhism, Sikhism, Hinduism, Judaism, Bahaism, Babism, Zoroastrianism, Mormonism, Jehovas Witnesses, Jainism, Confucianism etc. And each of these religions claim that their scripture is preserved from the day it was revealed (written) until our time. A religious belief is as authentic as the authenticity of the scripture it follows. And for any scripture to be labeled as authentically preserved it should follow some concrete and rational criteria.

God revealed it

"Those who disbelieve in the Reminder when it comes unto them (are guilty), for indeed it is an unassailable Scripture. Falsehood cannot come at it from before it or behind it. (It is) a revelation from the Wise, the Owner of Praise" (Qur’an 41:41-42).

Humanity has received the Divine guidance through two channels: firstly the word of Allah, secondly the Prophets who were chosen by Allah to communicate His will to human beings.